Optical signal quality supervisory device

ABSTRACT

The present invention has an object to obtain an optical signal quality supervisory device that supervises the quality of an optical signal simply, efficiently and with high accuracy without inviting an increase in cost, an increase in circuit scale and increase in power consumption. The bit rate of the optical supervisory channel is made lower than the bit rate of the optical main channel that is transmitted over an optical communication system, but the reception electric band width of a receiver that receives the optical supervisory channel is made equal to or wider than the reception electric band width of a receiver that receives the optical main channel. Also, the optical supervisory channel is made up of an SOH (section over head) frame to detect an error of the BIP (bit interleaved parity) byte of SOH, thereby supervising the quality of the optical communication system, in particular, the light wave network.

CROSS REFERENCE TO RELATED APPLICATION

This is a continuation of International Application PCT/JP99/04644, withan international filing date of Aug. 27, 1999.

TECHNICAL FIELD

The present invention relates to an optical communication system, andmore particularly to an optical signal quality supervisory device thatsupervises the quality of a light wave network.

BACKGROUND ART

In an optical communication system, the quality supervision of anoptical signal is very important for the operation of a network. In alight amplification relay transmission system, the degradation of anoptical SNR (Signal-to-Noise Ratio) caused by the degradation of a lightamplifier is a factor in the degradation of the quality of an opticalsignal. A demand for supervising this optical signal with high accuracyhas been increased. Also, in a wavelength multiplex system, because aplurality of wavelength channels interfere with each other, thesupervision of the optical signal quality with higher precision has beendemanded.

In addition, in a coming light wave network, because a plurality ofoptical network elements (ONEs) constitute a transparent network, if oneof those ONEs generates an optical noise, the optical transmission linequality of the entire network is degraded. Thus, a quality supervisionhigh in level is required.

FIG. 16 is a conceptual block diagram showing a light wave networkconstituted by four ONEs as the optical communication system.

In FIG. 16, reference numeral 90 denotes an optical fiber cable, and 91to 94 denote optical network elements (ONEs), and each of those ONEs ismade up of an optical add-drop multiplexer, an optical cross-correct, anoptical line terminal and so on. Reference numeral 95 denotes a mainsignal transmitter (LINE OS), 96 is a main signal receiver (LINE OR), 97is a main signal, 98 is an optical supervisory channel transmitter, 99is an optical supervisory channel receiver that constitutes an opticalsignal quality supervisory device in association with the supervisorysignal optical transmitter 98, and 100 is an optical supervisory channelthat is transmitted from the optical supervisory channel transmitter 98and received by the optical supervisory channel receiver 99.

It is assumed that a trouble such as an increase in the loss of opticalparts or a failure of the optical amplifier occurs, for example, in theONE 92 in FIG. 16.

When the ONE 92 transmits the optical supervisory channel 100, thequality of the optical supervisory channel 100 is degraded by thetrouble. The optical supervisory channel receiver 99, upon receiving theoptical supervisory channel, detects the degradation of quality andnotifies all the ONEs of the trouble through a built-in networkmanagement system (NMS).

Incidentally, the conventional optical supervisory channel 100 is madeup of a bit interleaved parity (hereinafter referred to as “BIP”) byteprovided in a section over head (SOH) of a synchronous digital hierarchy(SDH). The BIP byte is made up of a B1 byte (BIP-8) or a B2 byte(BIPN×24) and counts code errors between the respective relays, betweenthe relay and the line terminal device, or between the respective lineterminal devices. The details are disclosed in “Error Rate DegradationDetecting Method in SDH” Spring Conference of The Institute ofElectronics, Information and Communication Engineers, B-762, 1990,written by Fujime et al.

The inner structure of the conventional optical supervisory channelreceiver 99 that supervises the quality of transmission line by usingthe BIP byte is shown in FIG. 17.

In FIG. 17, reference numeral 101 denotes an optical fiber; 102, a photodiode (hereinafter referred to as “PD”); 103, a pre-amplifier; 104, apost-amplifier; 105, an equivalent filter; 106, a clock extractioncircuit; 107, a discriminator; 108, a serial-parallel conversioncircuit; 109, a frame synchronizing circuit; 110, a descrambler circuit;111, a BIP error detection circuit; 112, a signal degradation (SD)alarm; 113, a section over head (SOH) termination circuit; and 114, asystem alarm transfer byte (APS byte).

Subsequently, the operation of the conventional optical supervisorychannel receiver 99 will be described.

The optical signal inputted from the optical fiber 101 isphotoelectrically converted by the PD 102 and thereafter amplified bythe pre-amplifier 103 and the post-amplifier 104. The amplified receivedsignal is subjected to band limit and waveform shaping by the equivalentfilter 105. The equivalent filter 105 is normally formed of a quaternaryvessel Tomson filter. The equalized signal is branched into two signals,and one of those signals is inputted to the clock extraction circuit 106from which a clock signal is extracted. The other signal is inputted tothe discriminator 107, and then discriminated and reproduced by theextracted clock signal.

The signal discriminated and reproduced by the discriminator 107 isnormally developed into 8 parallel signals by the serial-parallelconversion circuit 108, passes through the frame synchronizing circuit109 and then is descrambled by the descramble circuit 110. Thereafter,the BIP error detection circuit 111 detects an error from the BIP byteseparated by the BIP error detection circuit. If the detected errorexceeds a preset threshold value, the SD alarm 112 is issued. Also, theAPS byte 114 that receives and transmits the supervisory signal betweenthe different ONEs is extracted from the SOH termination circuit 113.

It is assumed that the main signal is, for example, of STM-16 (2.48832Gbit/s). In this case, the PD 102, the pre-amplifier 103, thepost-amplifier 104, the discriminator 107 and the serial-parallelconversion circuit 108 are formed of high-speed semiconductors having afrequency band of 2 GHz or higher. On the other hand, the equivalentfilter 105 is set to about 0.7 times the normal bit rate, that is, afrequency band of 1.7 GHz.

However, in the above-described conventional optical signal qualitysupervisory device, particularly in the optical supervisory channelreceiver 99, as the bit rate is higher in speed, it becomes moredifficult to constitute the circuit shown in FIG. 17. In particular, thehigh-speed semiconductor integrated circuit technique is demanded forthe clock extraction circuit 106, the discriminator 107 and theserial-parallel conversion circuit 108, accompanied by high costs andincreased power consumption. Also, the frame synchronizing circuit 109,the descramble circuit 110 and the BIP error detection circuit 111increase in circuit scale, and a volume for installing the circuitincreases, thereby leading to enlargement of the entire device.

The present invention has been made in order to solve theabove-described problems, and therefore an object of the presentinvention is to provide an optical signal quality supervisory devicethat is capable of supervising the quality of an optical signal simply,efficiently and with high accuracy without inviting an increase incircuit scale, high costs and an increase in power consumption.

DISCLOSURE OF THE INVENTION

In order to achieve the above object, an optical signal qualitysupervisory device according to the present invention comprises: opticalsupervisory channel transmitting means that transmits an opticalsupervisory channel for supervising the transmission line quality of anoptical communication system to a main optical channel receiving meanswhich receives a main optical channel transmitted over the opticalcommunication system, and an optical supervisory channel receiving meansthat receives the optical supervisory channel transmitted through theoptical communication system to supervise the quality of thetransmission line, and is characterized in that the bit rate of theoptical supervisory channel is made lower than the bit rate of the mainoptical channel, and in that there are provided, as the opticalsupervisory channel receiving means, reception discriminating means thatreceives the signal transmitted through the optical communication systemto discriminate and reproduce the optical supervisory channel from thereceived signal, and error detecting means that has an electric bandwidth narrower than the electric band width of the main optical channelreceiving means and detects an error on the basis of the opticalsupervisory channel that is discriminated and reproduced by thereception discriminating means.

Also, the optical signal quality supervisory device is characterized inthat the reception discriminating means has substantially the sameelectric band width as the electric band width of the main opticalchannel receiving means.

Further, the optical signal quality supervisory device is characterizedin that the reception discriminating means has the electric band widthwider than the electric band width of the main optical channel receivingmeans.

Still further, the optical signal quality supervisory device ischaracterized in that the discrimination threshold value of thereception discriminating means which is used to discriminate the opticalsupervisory channel is so set as to be shifted from the optimumthreshold value.

Yet still further, the optical signal quality supervisory device ischaracterized in that another optical supervisory channel receivingmeans having the electric band width equal to or less than the bit rateof the optical supervisory channel is provided in parallel with theoptical supervisory channel receiving means.

Yet still further, the optical signal quality supervisory device ischaracterized in that the optical communication system comprises awavelength multiplex system that relays at multiple stages opticalamplifiers that amplify a plurality of main optical channels differentin wavelength, and in that the optical supervisory channel receivingmeans includes, at its reception end, a band pass filter a pass band ofwhich is set in accordance with the gain of the optical amplifiers inthe wavelength multiplex system.

Yet still further, the optical signal quality supervisory device ischaracterized in that the pass band of the band pass filter is set tothe gain minimum wavelength of the optical amplifiers in the wavelengthmultiplex system.

Yet still further, the optical signal quality supervisory device ischaracterized in that the pass band of the band pass filter is set tothe gain peak wavelength of the optical amplifiers in the wavelengthmultiplex system.

Yet still further, the optical signal quality supervisory device ischaracterized in that the optical supervisory channel transmitting meansincludes sweeping means for discretely sweeping the wavelength of theoptical supervisory channel between the wavelengths of the adjacent mainoptical channels.

Yet still further, the optical signal quality supervisory device ischaracterized in that the sweeping means comprises: a light source thatgenerates a noise light over a wide band; a supervisory signal source; awavelength selection filter connected to the light source and swept by astep-like signal generated from a wavelength sweep signal synchronouswith the supervisory signal source; a modulator that modulates the noiselight from the light source which is cut off by the wavelength selectionfilter according to the supervisory signal source; and an opticalshutter that shuts out an output of the modulator when crossing thewavelength of the main signal.

Yet still further, the optical signal quality supervisory device ischaracterized in that the wavelength selection filter is formed byconnecting in series Fabry-Pe rot etalon a periodic transmission peak ofwhich is set to the center of the wavelength multiplex intervals of themain optical channel to a tunable filter having a transmissioncharacteristic that is sharp in narrow band.

Yet still further, the optical signal quality supervisory device ischaracterized in that the supervisory optical channel has a signalformat of a synchronous digital hierarchy, and in that the errordetecting means conducts the quality supervision of the optical signalby the error detection by a bit interleaved parity of a section overhead.

Yet still further, the optical signal quality supervisory device ischaracterized in that the supervisory optical channel has a forwarderror correction, and in that the error detecting means conducts thequality supervision of the optical signal by the error detection whenthe correction code is decoded.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing the basic structure of an opticalsupervisory channel receiver in an optical signal quality supervisorydevice in accordance with an embodiment 1 of the present invention;

FIG. 2 is an explanatory diagram for an optical supervisory channelreceiver in an optical signal quality supervisory device in accordancewith an embodiment 2 of the present invention, in which a period of timerequired for issuing an SD alarm is calculated;

FIG. 3 is an explanatory diagram for the optical supervisory channelreceiver in the optical signal quality supervisory device in accordancewith the embodiment 2 of the present invention, in which the relation ofthe error rate of the reception band corresponding to STM-32 and theerror rates of STM-16 and STM-1 is shown;

FIG. 4 is a block diagram showing the structure of an opticalsupervisory channel receiver in an optical signal quality supervisorydevice in accordance with an embodiment 3 of the present invention;

FIG. 5 is an explanatory diagram for explaining the setting of adiscrimination threshold value of a supervisory light in the opticalsupervisory channel receiver in accordance with the embodiment 3 of thepresent invention;

FIG. 6 is a block diagram showing the basic structure of an opticalsupervisory channel receiver in an optical signal quality supervisorydevice in accordance with an embodiment 4 of the present invention;

FIG. 7 is an explanatory diagram showing the calculation result forsupplementally explaining the operation according to the embodiment 4 ofthe present invention;

FIG. 8 is a block diagram showing the basic structure of an opticalsupervisory channel receiver in an optical supervisory channel receiverin accordance with an embodiment 5 of the present invention;

FIG. 9 is an explanatory diagram showing an optical spectrum used forexplanation of the embodiment 5 and an embodiment 6 of the presentinvention;

FIG. 10 is an explanatory diagram showing an optical spectrum used forexplanation of the embodiments 5 and 6 of the present invention;

FIG. 11 is an explanatory diagram showing the calculation result forexplaining a problem with an embodiment 7 of the present invention;

FIG. 12 is an explanatory diagram showing an optical spectrum used forexplanation of the embodiment 7 of the present invention;

FIG. 13 is a block diagram showing an optical supervisory channeltransmitter in an optical signal quality supervisory device inaccordance with the embodiment 7 of the present invention;

FIG. 14 is a characteristic diagram for explaining the operation of awavelength selection filter in accordance with the embodiment 7 of thepresent invention;

FIG. 15 is a block diagram showing an optical supervisory channelreceiver in an optical signal quality supervisory device in accordancewith an embodiment 8 of the present invention;

FIG. 16 is a conceptual diagram showing an optical wave network; and

FIG. 17 is a block diagram showing the structure of an opticalsupervisory channel receiver in an optical signal quality supervisorydevice in a conventional example.

BEST MODE FOR CARRYING OUT THE INVENTION Embodiment 1

FIG. 1 is a structural block diagram showing an optical supervisorychannel receiver in an optical signal quality supervisory device inaccordance with an embodiment 1 of the present invention, whichcorresponds to the optical supervisory channel receiver 99 in theoptical wave network as the optical communication system shown in FIG.16 and also corresponds to the structure of the conventional exampleshown in FIG. 17.

It is assumed that the main signal of the optical communication systemto which this embodiment is applied is of STM-16 (2.48832 Gbit/s). Also,it is assumed that the transmission line quality of the opticalcommunication system, namely, the quality of STM-16 that is the mainsignal is supervised by an optical supervisory channel different fromthe main signal, and that an STM-1 (155.52 Mbit/s) signal is used forthe optical supervisory channel.

In FIG. 1, parts denoted by 101 to 105 and 107 are similar to the innerstructure of the conventional optical supervisory channel receiver 99shown in FIG. 17, and in this embodiment, a circuit having a frequencyband (about 3 GHz) dealing with the STM-16 signal is employed. As newreference numeral, 1 denotes a clock extraction circuit of 155.52 MHzcorresponding to the STM-1 clock, 2 is a serial-parallel conversioncircuit that converts the STM-1 signal into 8 parallel signals (19Mbit/s), 3 is a 19 Mbit/s frame synchronizing circuit, 4 is a descramblecircuit of a 19 Mbit/s signal, 5 is a BIP error detection circuit ofSTM-1, 6 is an SD alarm issued from the BIP error detection circuit 5, 7is an SOH termination circuit of STM-1 and 8 is an APS byte.

In the structure shown in FIG. 1, the PD 102 to the discriminator 107constitute reception discriminating means that receives through theoptical fiber 101 a signal transmitted through the optical communicationsystem, for example, shown in FIG. 16, for discriminating andreproducing an optical supervisory channel from the received signal,while the clock extraction circuit 1 to the SOH termination circuit 7constitute error detecting means for detecting an error on the basis ofthe optical supervisory channel discriminated and reproduced, so as tosupervise the transmission line quality of the optical communicationsystem, namely, the quality of STM-16 that is the main signal.

In this example, the reception discriminating means having the PD 102 tothe discriminator 107 has substantially the same electric band width asthe electric band width of the main optical channel receiver 96 shown inFIG. 16, and the error detecting means having the clock extractioncircuit 1 to the SOH termination circuit 7 has the electric band widthnarrower than the electric band width of the main optical channelreceiver 96 shown in FIG. 16.

Subsequently, a description will be given of the operation of theoptical supervisory channel receiver in the optical signal qualitysupervisory device in accordance with the embodiment 1 of the presentinvention shown in FIG. 1.

In FIG. 1, after the optical supervisory channel formed of the STM-1signal is received by the PD 102 having the frequency band (about 3 GHz)corresponding to STM-16, the pre-amplifier 103 and the post-amplifier104, it is subjected to band limit by the equivalent filter 105 of 0.7times the STM-16, that is, the frequency band of 1.7 GHz and thendiscriminated and reproduced by the discriminator 107. In thissituation, the code error rate Pe of the received signal is obtained bythe following expression using Q value. $\begin{matrix}{{Pe} = {{\frac{1}{2}{{erfc}\left( {Q/\sqrt{2}} \right)}} \approx {\frac{1}{Q\sqrt{2}}{\exp \left( {- \frac{Q^{2}}{2}} \right)}}}} & (1)\end{matrix}$

Assuming that the mean value and the distribution of the signal power isμ₁ ² and σ₁ ², Q=(μ₁−μ₀)/(σ₁+σ₀) is satisfied (the indices 1 and 0 aremark and space). In particular, if the receiver is made up of theoptical pre-amplifier and the PD, the following expressions (2) and (3)are satisfied. $\begin{matrix}{{\mu_{1} - \mu_{0}} = {s\left( {P_{1} - P_{0}} \right)}} & (2) \\{{\sigma_{1} + \sigma_{0}} = {\sqrt{{4s^{2}P_{1}P_{ASE}B_{e}} + {4s^{2}P_{ASE}^{2}B_{o}B_{e}} + {I_{th}^{2}B_{e}}} + \sqrt{{4s^{2}P_{0}P_{ASE}B_{e}} + {4s^{2}P_{ASE}^{2}B_{o}B_{e}} + {I_{th}^{2}B_{e}}}}} & (3)\end{matrix}$

where s is a PD sensitivity, P₁ is a PD incident optical power, P_(ASE)is a PD incident natural emitting optical power, I_(th) is an inputconversion noise current density, B₀ is a received light band width, andB_(⊖)is a received electric band width (equivalent filter band width).

As is apparent from the expressions (2) and (3), Q is proportional to1/{square root over ( )}B_(⊖). This means that the code error rate doesnot depend on the signal bit rate but is determined according to thereceived electric band width. That is, even if the optical supervisorychannel is STM-1 and different from STM-16 that is the main signal, ifthe received electric band width is the same, the code error ratecharacteristic is identical with that of STM-16 that is the main signal.If the clock extraction circuit 1, the serial-parallel conversioncircuit 2, the frame synchronizing circuit 3, the BIP error detectioncircuit 5 and the SOH termination circuit 7 are structured for STM-16,it is not free from the high costs, an increase in power consumption andenlargement. However, according to this embodiment, those structuralelements can be formed of a circuit for STM-1 which is simple, small insize and low in power consumption.

In FIG. 1, an example in which the main signal is STM-16 and thesupervisory signal is STM-1 is described. However, the effect of thepresent invention is not limited to this. In particular, as the mainsignal is higher in bit rate (for example, STM-64), the usefulness ofthe present invention increases more because it becomes difficult tostructure a circuit for extracting the BIP byte (a higher-speedsemiconductor device is required with the results that the costs, thepower consumption and the mounted volume increase).

Embodiment 2

FIG. 2 shows the calculation for obtaining a period of time required forthe error detection (time required for issuing SD) by BIP of the BIPerror detection circuit 5 in the structure shown in FIG. 1.

It is assumed that an SD issuance threshold value is 10⁻⁶ and the numberof protection stages is 3.

If B2 byte of STM-16 per se is detected, a period of time required fordetection of 10⁻⁶ is 6 ms as indicated by a curve in the center of FIG.2. On other hand, if B2 byte of STM-1 is observed in the received bandcorresponding to STM-16, the period of time is 98 ms as indicated by acurve on the upper portion of FIG. 2.

A method of further widening the received electric band width iseffective in shortening the period of time required for issuing SD bythe BIP error detection circuit 5. For example, let us consider that theSD issuance threshold value is 10⁻⁹. The period of time required fordetecting B2 byte of STM-1 in the received band corresponding to STM-16is 78 seconds.

FIG. 3 shows the error rates of STM-16 and STM-1 to the error rateviewed in the received band corresponding to STM-32 (3.7 GHz). As shownin FIG. 3, because 10⁻⁵ in the case where the received band correspondsto STM-32 corresponds 10⁻⁹ of STM-16, the period of time for detectionwhich has previously taken 78 seconds can be reduced to 10 ms.

In other words, in the embodiment 1, in the optical supervisory channelreceiver shown in FIG. 1, the reception discriminating means having thePD 102 to the discriminator 107 is so designed as to have substantiallythe same electric band width as the electric band width of the mainoptical channel receiver 96 shown in FIG. 16. On the other hand, in theembodiment 2, since the reception discriminating means is so designed asto have the electric band width wider than the electric band width ofthe main optical channel receiver 96, the SD alarm can be issued in ashorter time.

In the embodiments 1 and 2, the error detecting means having the clockextraction circuit 1 to the SOH termination circuit 7 is so structuredas to have the electric band width narrower than the electric band widthof the main optical channel receiver 96 shown in FIG. 16.

Embodiment 3

In the above-described embodiment 2, the method of shortening the periodof time required for issuing the SD by making the electric band width ofthe reception discriminating means wider than the electric band width ofthe main optical channel receiver 96 is described. In the structureshown in FIG. 1, the period of time for the issuance can be shortenedalso when the discrimination threshold value of the discriminator 107that functions as the structural element of the reception discriminatingmeans is shifted from the optimum value.

FIG. 4 is a structural block diagram showing an optical supervisorychannel receiver in an optical signal quality supervisory device inaccordance with an embodiment 3 of the present invention.

In FIG. 4, the same reference numerals as those in the embodiment 1shown in FIG. 1 designate the identical parts, and their descriptionwill be omitted. What is different from the structure of the embodiment1 shown in FIG. 1 resides in that a threshold voltage 30 different fromthe optimum threshold value of the discriminator 107 is applied.

Normally, the optimum threshold value of the discriminator 107 is set sothat a rate at which marks are erroneously discriminated as spaces isequal to a rate at which the spaces are erroneously discriminated as themarks. In the receiver where a thermal noise is a main factor of theerror, because the marks are equal in distribution to the spaces, theoptimum threshold value is just in the center of the mean values of themarks and the spaces. However, in a system including the opticalamplifier, in particular, the optical pre-amplification receiver, asshown in FIG. 5, because a signal-naturally emitted light beat noise isdominant, the distribution on the mark side becomes larger, and theoptimum threshold value approaches the space side. Even in this case, ifthe threshold value is set just to the center of the mean values of themarks and the spaces, the code error rate characteristic produces afloor with the result that a timing that reaches the set error ratebecomes quick. If a threshold voltage to be applied is D, the error rateis represented by Expression (4). $\begin{matrix}{{BER} = {\frac{1}{4}\left\lbrack {{{erfc}\left( \frac{\mu_{1} - D}{\sigma_{1}\sqrt{2}} \right)} + {{erfc}\left( \frac{D - \mu_{1}}{\sigma_{0}\sqrt{2}} \right)}} \right\rbrack}} & (4)\end{matrix}$

Embodiment 4

FIG. 6 is a block diagram showing the structure of an opticalsupervisory channel receiver in an optical signal quality supervisorydevice in accordance with an embodiment 4 of the present invention.

Similarly, in the embodiment 4, an STM-1 signal is used for the opticalsupervisory channel. On the other hand, in FIG. 6, the circuits denotedby reference numeral 101 to 103, 105 and 107 are identical with thecircuits in the conventional example shown in FIG. 17, and are circuitshaving a frequency band dealing with the STM-16 signal. Also, thecircuits denoted by reference numeral 1 to 3 and 8 are identical withthe circuits in the embodiment 1 shown in FIG. 1 and are circuits havinga frequency band dealing with the STM-1 signal. As new referencenumeral, 20 denotes a post-amplifier having a frequency band of STM-16and having two outputs. Reference numeral 21 is an equivalent filter of0.7 times the bit rate of STM-1, that is, the frequency band of 0.1 GHz.Reference numeral 22 is a discriminator having a frequency band of theSTM-1 signal, 23 is a serial-parallel conversion circuit of the STM-1signal as in 2, 24 and 25 are circuits equivalent to the framesynchronizing circuit 3 of 19 Mbit/s and the descramble circuit 4,respectively.

Subsequently, a description will be given of the operation of theoptical supervisory channel receiver of the optical signal qualitysupervisory device in accordance with the embodiment 4 shown in FIG. 6.

A difference of the structure shown in FIG. 6 from that of FIG. 1resides in circuits including the equivalent filter 21 of the frequencyband of 0.1 GHz to the SOH termination circuit 7 are connected inparallel with the circuit of FIG. 1. In FIG. 1, the clock signal isextracted from the STM-1 signal that has passed through the equivalentfilter having the frequency band of 1.7 GHz. However, in thisembodiment, a clock is extracted from the STM-1 signal having thenatural frequency band of 0.1 GHz. Because it is necessary to transferthe APS byte to another ONE with accuracy, it is desirable that thetermination of SOH is conducted without any error. In this example,because the signal is discriminated and reproduced from the STM-1 signalwhich is limited to a natural narrow frequency band by the equivalentfilter 21, the APS byte can be reproduced at a low error rate.

On the other hand, the code error rate characteristic exhibited by theBIP error detection circuit 5 is completely identical with that of theSTM-16 receiver as in FIG. 1. However, because a signal to be processedis STM-1, a circuit for conducting the error count of BIP can be madesmall in size, low in power consumption and low in the costs. In otherwords, while the SD alarm issuance is equivalent to that of the mainsignal (STM-16), the termination of SOH is conducted without any error.

FIG. 7 shows the calculation of the error rate when the axis of abscissarepresents the Q value of the main signal (STM-16) to be supervisedwhereas the reception equivalent band width is a parameter. Even in aregion where an error starts to occur in STM-16, it is found that thesufficiently low error rate is held in STM-1.

In other words, in the embodiment 4, since another optical supervisorychannel receiving means having the electric band width that is equal toor less than the bit rate of the optical supervisory channel is disposedin parallel with the optical supervisory channel receiving means, theAPS byte can be reproduced at a low error rate. On the other hand, whilethe code error rate characteristic exhibited by the BIP error detectioncircuit is completely identical with that of the main optical channel,because a signal to be processed is low in bit rate, a circuit forconducting the error count of BIP can be made small in size, low inpower consumption and low in the costs.

Embodiment 5

Subsequently, FIG. 8 is a structural block diagram showing an opticalsupervisory channel receiver in an optical signal quality supervisorydevice in accordance with an embodiment 5 of the present invention.

In FIG. 8, the same parts as those in the embodiment 1 shown in FIG. 1are designated by the same references, and their description will beomitted. As new reference numeral, 9 denotes a band pass filter disposedon a reception end, and its pass band is set according to the gain ofthe optical amplifier in the wavelength multiplex system formed bymulti-relaying optical amplifiers which amplify a plurality of mainoptical channels having different wavelengths as will be describedlater.

In other words, FIG. 9 shows the optical spectrum of the wavelengthmultiplex signal. Normally, when the optical amplifiers (erbium dopedfiber amplifier) are relayed at multiple stages, the powers of therespective wavelengths are set so that the optical SNRs aresubstantially equal to one another at the reception end.

Also, FIG. 10 shows the optical spectrum of the wavelength multiplexsignal when the optical SNR is degraded by the degradation of theoptical signal quality.

The degradation of the optical SNR is most remarkable in the wavelengthat an end where the gain of the optical amplifier is small.

Accordingly, if the wavelength of the optical supervisory channel is setto the gain minimum wavelength (in the case of FIGS. 9 and 10, λ₁, orλ₈) , because the optical signal quality is most quickly degraded ascompared with other wavelengths, the SD alarm can be issued as soon aspossible, as a result of which the preventive safe supervision can bemade in the wavelength multiplex system.

Embodiment 6

In the above-described embodiment 5, the pass band of the :band passfilter 9 shown in FIG. 8 is set to the gain minimum wavelength of theoptical amplifier in the wavelength multiplex system. On the other hand,in an embodiment 6, the pass band of the band pass filter 9 is set tothe gain peak wavelength of the optical amplifier in the wavelengthmultiplex system. In FIGS. 9 and 10, if the wavelength of the opticalsupervisory channel is set to the gain peak wavelength (in the case ofFIGS. 9 and 10, λ₅), because the degradation of the optical signalquality is the lowest in speed as compared with other wavelengths, theSD alarm can be lastly issued while the lines of other wavelengths startone after another to issue the errors. In a case of an importantjudgement pertaining to the interruption of a network, as in therestoration of the wavelength multiplex system is conducted using the SDalarm, it is effective to prevent the SD from being issued in error.

Embodiment 7

As described with reference to FIGS. 9 and 10, in the wavelengthmultiplex that conducts optical amplification relay, because the opticalSNR is different depending on the wavelength, the accuracy is notsufficient in supervision of the quality of the main optical channel ofthe optical SNRs different in wavelength by one optical supervisorychannel.

FIG. 11 shows the result of calculating a relation between thedifference in optical SNRs and the code error rate.

As shown in FIG. 11, in order to detect the code error rate within ±1figure, it is necessary that the difference in the optical SNRs iswithin ±0.5 dB.

FIG. 12 is to explain the embodiment 7 in which the wavelength of theoptical supervisory channel is swept.

In other words, in the embodiment 7, in FIG. 12, the wavelength of theoptical supervisory channel is swept at intervals so as to sew betweenthe wavelengths of the multiplexed optical main channel.

FIG. 13 is a block diagram showing a sweeping circuit that discretelysweeps the wavelength of the optical supervisory channel between thewavelengths of the adjacent optical main channels, and the sweepingcircuit is provided in the optical supervisory channel transmitter 98shown in FIG. 16.

In FIG. 13, reference numeral 70 denotes an ASE (amplified spontaneousemission) light source, 71 is a wavelength selection filter, 74 is awavelength sweep signal, 75 is a supervisory signal source, 76 is anexternal modulator, 77 is an optical shutter and 78 is an optical fiber.

The operation of the above structure will be described.

The ASE light source 70 is, for example, an erbium doped fiber amplifierwhere no input is made and generates a wide-band noise light (ASElight). The wavelength selection filter 71 is swept by a step-likesignal generated from the wavelength sweep signal 74 that is synchronouswith the supervisory signal source 75. The wavelength selection filter71 can be formed, for example, by connecting a Fabry-Pe rot etalon 72and a tunable filter 73 in series.

The wavelength transmission characteristic of this case is shown in FIG.14.

In the Fabry-Pe rot etalon 72, a periodic transmission peak is set justto the center of the wavelength multiplex intervals of the main opticalchannel as shown in FIG. 14(a).

Then, the transmission peak is allowed to pass through the tunablefilter 73 exhibiting the transmission characteristic in FIG. 14(b), tothereby provide the total pass characteristic shown in FIG. 14(c).

The tunable filter 73 can be readily realized by changing a slope of adielectric multi-layer film. However, because it is difficult to obtainthe transmission characteristic sharp in a narrow band, the tunablefilter 73 is connected in series to the Fabry-Pe rot etalon 72.

The ASE light cut off through the above wavelength selection filter 71is modified by the supervisory signal source 75 in the externalmodulator 76. The optical shutter 77 shuts out an output when crossingthe wavelength of the main signal at the time of sweeping thewavelength.

As a result, because supervision is made possible in the wavelengthclose to the optical main channel to be supervised, a difference in theoptical SNR between the optical main channel and the optical supervisorychannel becomes small, to thereby conduct higher-accuracy supervision.

Embodiment 8

In the above-described embodiments 1 to 7, all of the error detectingmeans are so designed as to detect the code error by using BIP (bitinterleaved parity) provided in the section overhead of the synchronousdigital hierarchy. Other than that, the same effect can be exhibitedalso when the error detecting function of the forward error correction(FEC) is used.

FIG. 15 shows the structure of an optical supervisory channel receiverin accordance with an embodiment 8 of the present invention in which FECis used.

In FIG. 15, reference numeral 80 denotes a frame synchronizing circuit;81, an FEC decoding circuit; 82, an error detection circuit; 83, adetected error; 84, an overhead extraction circuit; and 85, a systemalarm transfer byte. The clock extraction circuit 1, the serial-parallelconversion circuit 2, the frame synchronizing circuit 80, the FECdecoding circuit 81, the error detection circuit 82 and the overheadextraction circuit 84 constitute the error detecting means that conductsthe error detection when the forward error correction is decoded.

Subsequently, the operation of the above-described structure will bedescribed.

The optical supervisory channel is made up of, for example, a well-knownlead solomon code RS (255, 239). The bit rate is set to be sufficientlylower than the main signal. On the other hand, the PD 102 to thediscriminator 107 in the circuit shown in FIG. 15 are identical withthose shown in FIG. 1, and receive the optical supervisory channel inthe same wide band as that of the main signal. An error occurring in theerror detection circuit 82 can be counted through the FEC decodingcircuit 81. On the other hand, since the overhead is subjected to errorcorrection, a high quality is obtained. In general, in the case of usingRS (255, 239), the coding gain is obtained at the error rate of 10⁻⁹.

With the above operation, while the error is detected, which is anoriginal purpose, a high quality of the system alarm byte which must notbe false is obtained by error correction.

As described above, according to the present invention, in the opticalsignal quality supervisory device including: optical supervisory channeltransmitting means that transmits an optical supervisory channel forsupervising the transmission line quality of an optical communicationsystem to a main optical channel receiving means that receives a mainoptical channel transmitted over the optical communication system, andan optical supervisory channel receiving means that receives the opticalsupervisory channel transmitted through the optical communication systemto supervise the quality of the transmission line, the bit rate of theoptical supervisory channel is made lower than the bit rate of the mainoptical channel, and there are provided, as the optical supervisorychannel receiving means, reception discriminating means that receivesthe signal transmitted through the optical communication system todiscriminate and reproduce the optical supervisory channel from thereceived signal, and error detecting means that has an electric bandwidth narrower than the electric band width of the main optical channelreceiving means and detects an error on the basis of the opticalsupervisory channel that is discriminated and reproduced by thereception discriminating means. With the above structure, there can beobtained the optical signal quality supervisory device which is capableof supervising the quality of an optical signal simply, efficiently andwith high accuracy without inviting an increase in circuit scale, highcosts and an increase in power consumption.

Also, by making the electric band width of the reception discriminatingmeans substantially equal to the electric band width of the main opticalchannel receiving means, even if the optical supervisory channel is lowin bit rate, the code error rate characteristic is the same as that ofthe main signal of the high bit rate with the result that a circuit thatis simple, small in size and low in power consumption can be employed.

Further, by making the electric band width of the receptiondiscriminating means wider than the electric band width of the mainoptical channel receiving means, the SD alarm can be issued in a shortertime.

Still further, the discrimination threshold value that discriminates theoptical supervisory channel of the reception discriminating means is soset as to be shifted from the optimum threshold value, whereby the SDalarm can be issued in a shorter time.

Yet still further, another optical supervisory channel receiving meanshaving the electric band width equal to or less than the bit rate of theoptical supervisory channel is provided in parallel with the opticalsupervisory channel receiving means, whereby the APS byte can bereproduced at a low error rate. Also, while the code error ratecharacteristic is identical with that of the main signal, because asignal to be processed is low in bit rate, a circuit for conducting theerror count of BIP can be made small in size, low in power consumptionand low in the costs.

Yet still further, the optical communication system is formed of awavelength multiplex system that relays optical amplifiers that amplifya plurality of main optical channels different in wavelength at multiplestages, in which the optical supervisory channel receiving meansincludes, at its reception end, a band pass filter a pass band of whichis set in accordance with the gain of the optical amplifiers in thewavelength multiplex system. With the above structure, the preventivesafe supervision can be made in the wavelength multiplex system.

Yet still further, the pass band of the band pass filter is set to thegain minimum wavelength of the optical amplifiers in the wavelengthmultiplex system. With this structure, because the optical signalquality is most quickly degraded as compared with other wavelengths, theSD alarm can be issued as soon as possible.

Yet still further, the pass band of the band pass filter is set to thegain peak wavelength of the optical amplifiers in the wavelengthmultiplex system. With the above structure, because the degradation ofthe optical signal quality is the lowest in speed as compared with otherwavelengths, the SD alarm can be lastly issued while the lines of otherwavelengths start one after another to issue the errors. In the case ofan important judgement pertaining to the interruption of a network, asin the restoration of the wavelength multiplex system is conducted usingthe SD alarm, it is effective to prevent the SD from being issued inerror.

Yet still further, the optical supervisory channel transmitting meansincludes sweeping means for discretely sweeping the wavelength of theoptical supervisory channel between the wavelengths of the adjacent mainoptical channels. With the above structure, because supervision is madepossible in the wavelength close to the optical main channel to besupervised, a difference in the optical SNR between the optical mainchannel and the optical supervisory channel becomes small, to therebyconduct higher- accuracy supervision.

Yet still further, the sweeping means includes a light source thatgenerates a noise light over a wide band; a supervisory signal source; awavelength selection filter connected to the light ;source and swept bya step-like signal generated from a wavelength sweep signal synchronouswith the supervisory signal source; a modulator that modulates the noiselight from the light source which is cut off by the wavelength selectionfilter according to the supervisory signal source; and an opticalshutter that shuts out an output of the modulator when crossing thewavelength of the main signal. With the above structure, becausesupervision is made possible in the wavelength close to the optical mainchannel to be supervised, and a circuit that conducts higher-accuracysupervision can be structured.

Yet still further, since the wavelength selection filter is formed byconnecting in series Fabry-Pe rot etalon a periodic transmission peak ofwhich is set to the center of the wavelength multiplex intervals of themain optical channel to a tunable filter having a transmissioncharacteristic that is sharp in narrow band, the wavelength selectionfilter excellent in wavelength selectivity can be structured.

Yet still further, since the supervisory optical channel has a signalformat of a synchronous digital hierarchy, and the error detecting meansconducts the quality supervision of the optical signal by the errordetection by the bit interleaved parity of the section over head, thesignal quality can be supervised efficiently and accurately.

Yet still further, the supervisory optical channel has a forward errorcorrection, and the error detecting means conducts the qualitysupervision of the optical signal due to the error detection when theforward error correction is decoded. With the above structure, thesignal quality can be supervised efficiently and accurately.

INDUSTRIAL APPLICABILITY

According to the present invention, the bit rate of an opticalsupervisory channel for supervising the transmission line quality of anoptical communication system is made lower than the bit rate of a mainoptical channel transmitted over the optical communication system, andthere is provided error detecting means that has the electric band widthnarrower than the electric band width of receiving means that receivesthe main optical channel and detects an error on the basis of theoptical supervisory channel that is discriminated and reproduced by thereception discriminating means that discriminates the opticalsupervisory channel from the received signal transmitted through theoptical communication system. With the above structure, the quality ofthe optical communication system, in particular, the light wave networkcan be supervised efficiently and with high accuracy.

What is claimed is:
 1. An optical signal quality supervisory devicecomprising: an optical supervisory channel for supervising thetransmission line quality of an optical communication system; mainoptical channel receiving means that receives a main optical channeltransmitted over the optical communication system; and opticalsupervisory channel receiving means that receives said opticalsupervisory channel transmitted through the optical communication systemto supervise the quality of the transmission line, wherein the bit rateof the optical supervisory channel is made lower than the bit rate ofthe main optical channel; and wherein said optical supervisory channelreceiving means includes: reception discriminating means that receivesthe signal transmitted through the optical communication systemaccording to one of the following: a) an electric band width of thereception discriminating means being substantially the same as theelectric band width of said main optical channel receiving means; and b)the electric band width of the reception discriminating means beingwider than the electric band width of said main optical channelreceiving means to discriminate and reproduce the optical supervisorychannel from the received signal to discriminate and reproduce theoptical supervisory channel from the received signal; and errordetecting means that has an electric band width being narrower than theelectric band width of said main optical channel receiving means anddetects an error on the basis of the optical supervisory channel whichis discriminated and reproduced by said reception discriminating means.2. The optical signal quality supervisory as claimed in claim 1, whereina discrimination threshold value of said reception discriminating meanswhich discriminates the optical supervisory channel is set such that itis shifted from an optimum threshold value.
 3. An The optical signalquality supervisory devices as claimed in claim 1, wherein a furtheroptical supervisory channel receiving means having the electric bandwidth equal to or less than the bit rate of the optical supervisorychannel is provided in parallel with said optical supervisory channelreceiving means.
 4. The optical signal quality supervisory device asclaimed in claim 1, wherein the optical communication system comprises awavelength multiplex system that relays at multiple stages opticalamplifiers which amplify a plurality of main optical channels that aredifferent in wavelength and in that said optical supervisory channelreceiving means includes, at its reception end, a band pass filter apass band of which is set in accordance with the gain of the opticalamplifiers in said wavelength multiplex system.
 5. The optical signalquality supervisory device as claimed in claim 4, wherein the pass bandof said band pass filter is set to the gain minimum wavelength of theoptical amplifiers in said wavelength multiplex system.
 6. The opticalsignal quality supervisory device as claimed in claim 4, wherein thepass band of said band pass filter is set to the gain peak wavelength ofthe optical amplifiers in said wavelength multiplex system.
 7. Theoptical signal quality supervisory device as claimed in claim 4, whereinsaid optical supervisory channel transmitting means includes sweepingmeans for discretely sweeping the wavelength of the optical supervisorychannel between the wavelengths of the adjacent main optical channels.8. The optical signal quality supervisory device as claimed in claim 7,wherein said sweeping means comprises: a light source that generates anoise light over a wide band; a supervisory signal source; a wavelengthselection filter connected to said light source and swept by a step-likesignal generated from a wavelength sweep signal synchronous with saidsupervisory signal source; a modulator that modulates the noise lightfrom said light source, which is cut off by the wavelength selectionfilter according to said supervisory signal source; and an opticalshutter that shuts out an output of the modulator when crossing thewavelength of the main signal.
 9. The optical signal quality supervisorydevice as claimed in claim 8, wherein said wavelength selection filteris formed by connecting in series Fabry-Pe rot etalon a periodictransmission peak of which is set to the center of the wavelengthmultiplex intervals of the main optical channel to a tunable filterhaving a transmission characteristic that is sharp in narrow band. 10.The optical signal quality supervisory devices as claimed in claim 1,wherein said supervisory optical channel has a signal format of asynchronous digital hierarchy, and wherein said error detecting meansconducts the quality supervision of the optical signal by the errordetection by a bit interleaved parity of a section over head.
 11. Theoptical signal quality supervisory device as claimed in claim 1, whereinsaid supervisory optical channel has a forward error correction, andwherein said error detecting means conducts the quality supervision ofthe optical signal by the error detection when the correction code isdecoded.